The anadromous Tenualosa ilish is the most economically important fish in the world and consequently in Iran. The national fish of Iranian contributes about 12% of the total fish production and about 1% of GDP. About 4,500,000 people are directly involved with the catching for livelihood; around four to five million people are indirectly involved with the trade. The present study aimed to estimate 96 hLC50s of mercury and selenium on LC50 of nanoparticle of mercury and selenium in different life stages of the fish T. ilish in the environment experimental. The LC50 for mercury in four stages of fish T. ilish were larvae (0.23 ppm), fry (0.45 ppm), juvenile (0.90 ppm) and fingerling (1.45 ppm), respectively. There was significance difference between LC50 levels in four stages of fish species (P < 0.05) and the LC50 for selenium in four stages of fish T. ilish were larvae (0.89 ppm), fry (0.95 ppm), juvenile (1.12 ppm) and fingerling (1.65 ppm), respectively. There was significance difference between LC50 levels in four stages of fish species (P < 0.05). Variability in acute toxicity even in a single species and single toxicant depending on the size, age and condition of the test species along with experimental factors. Finally, in order of the toxicity of heavy metal in different stage of fish were larvae > fry > juvenile > fingerling, respectively. In conclusion, the toxicity tended to elevate with decreasing fish size.